The invention relates to a method of packaging a micro-mechanical systems (MEMS) device in a packaging atmosphere having a high thermal conductivity, and to a packaged MEMS device. The invention enables the MEMS device to accommodate higher power dissipation, to operate in a higher temperature environment than conventionally packaged devices and is capable of increasing the operational lifetime of the MEMS device.
Micro-mechanical systems (MEMS) devices are key components in photonic switches which switch optical signals at the optical layer. MEMS mirrors are used in photonic switches to reflect optical signals between input and output ports. A photonic switch usually consists of an array of MEMS mirrors. Each mirror is movable, for example in response to an electrical input, to switch incident light in direction by reflection from a mirror surface. The optical signals propagate in free space (as light beams) within the switch, and the MEMS mirrors are positioned by means of actuators so as to intercept the beams.
Depending on the actuators used, MEMS mirror devices fall into elastomeric, membrane and beam type devices. Beam type MEMS mirrors include torsion beams, cantilever beams and flexure beam devices. These beam type devices consist generally of a relatively thick reflector component mounted on an actuator which comprises a relatively thin beam. The mirror is moved into position by actuating the beam, for example, using electrical means.
MEMS devices are subject to mechanical failure problems as well as general erosion problems associated with deterioration of the reflective surface of the MEMS mirror. Both types of problems can be exacerbated by heat.
A MEMS mirror device is generally reflective only on one surface to enable heat to be dissipated from the non-reflective surface. The reflective surface is subject to long term degradation (wearout) through normal use and from sudden catastrophic failure through overloading a which can affect either the actuator motion or damage the mirror""s reflective coating.
Normally, a temperature rise is generated by absorption of a small portion of the incident optical power by the mirror""s reflective surface. The energy absorbed enhances the mobility of the atoms and molecules making up the mirror surface, and increases diffusion rates both tangentially across the mirror surface and into the mirror substrate. Any increase in temperature greatly increases the rate of these diffusion processes, promoting more rapid surface roughening and general degradation of the mirror""s reflective coating.
At least two underlying mechanisms can affect the optical power range a mirror is capable of handling. Firstly, catastrophic failure when, for example, an extremely intense optical beam instantaneously damages the reflective coating due to thermal overstress. Secondly, slow degradation in the reflective surface due to sustained heating caused by absorption of optical power. Once the mirror coating has deteriorated to below an acceptable level, the whole MEMS device must be replaced. To ensure that the working lifetime of a MEMS mirror is acceptably long, it is desirable to set a limit on the power of the optical beams the mirror reflects.
It is known that the rate of degradation of a mirror surface is heat-dependent, the rate of degradation increasing as the temperature increases. The degradation of a reflective coating of a MEMS mirror surface can be reduced by increasing the efficiency of heat removal from the mirror coating, however, design constraints make this difficult to implement. Degradation arises from diffusion either tangentially or across the mirror coating layer into the mirror substrate. Diffusion induced damage can increase both the surface roughness and reduce the reflectivity of the mirror surface, and it is highly desirable to minimise diffusion in a MEMS mirror device.
Despite the effect that a build up of heat within a MEMS device has on increasing the rate of diffusion and accelerating the deterioration of the reflective mirror coating, it is difficult to provide safety mechanisms to limit heat building up during normal operational use of the MEMS device, much less control catastrophic power surges that may lead to more sudden failure.
To protect a MEMS device from moisture and contamination, it is known to hermetically seal the MEMS device within suitable packaging. Conventional methods includes simply sealing a MEMS device within an air atmosphere or selecting a nitrogen based packaging atmosphere. These atmospheres can inhibit oxidation and provides protection against moisture. Conventional packaging techniques select packaging atmospheres principally to provide some protection against moisture, and do not consider the thermal characteristics of the atmosphere.
One object of the invention seeks to mitigate or obviate the above disadvantages by providing a method of packaging a MEMS device in an atmosphere having a relatively high thermal conductivity compared to conventional packaging atmospheres.
Another object of the invention seeks to provide a MEMS device packaged in an atmosphere having a relatively high thermal conductivity compared to conventional packaging atmospheres.
Yet another object of the invention seeks to provide a packaging atmosphere having a relatively high thermal conductivity compared to conventional packaging atmosphere. The packaging atmosphere improves the level of thermal protection for a packaged MEMS device over that provided by conventional means.
A first aspect of the invention relates to a packaging atmosphere packaging a MEMS device, the packaging atmosphere selected to have a thermal conductivity exceeding the thermal conductivity of air. Preferably, the MEMS device has at least one thermally dependent characteristic, more preferably, at least one said thermally dependent characteristic affects the functionality of the MEMS device.
Preferably, the packaging atmosphere provided enables the MEMS device to handle a higher level of optical power than the level of optical power the MEMS device can handle when packaged in an atmosphere taken from the group of: air and nitrogen.
Preferably, the packaging atmosphere improves the thermal stability of the device.
Preferably, the packaging atmosphere has a thermal conductivity exceeding 0.03 Wmxe2x88x921Kxe2x88x921.
Preferably, the packaging atmosphere includes helium.
Preferably, the thermal efficiency of the packaging atmosphere exceeds the thermal efficiency of air by a factor of 1.5.
A second aspect of the invention relates to a method of packaging a MEMS device, comprising:
a. selecting a packaging atmosphere which improves the thermal stability of the device; and
b. hermetically sealing the MEMS device within the packaging atmosphere.
Preferably, the device is sealed in an packaging atmosphere having a thermal conductivity exceeding the thermal conductivity of air.
Preferably, the device is sealed in a packaging atmosphere having a thermal conductivity exceeding 0.03 Wmxe2x88x921Kxe2x88x921.
Preferably, the packaging atmosphere is selected to have composition including a predetermined proportion of helium.
Preferably, the selected packaging atmosphere is helium.
A third aspect of the invention relates to a packaged MEMS device, the MEMS device having thermally dependent characteristics affecting its functionality, the packaged MEMS device comprising: a MEMS device surrounded by an packaging atmosphere and sealed within the packaging atmosphere by a packaging material, the packaging atmosphere having a thermal conductivity exceeding the thermal conductivity of air.
Preferably, the packaging atmosphere extends the range over which the device is thermally stable.
Preferably, the packaging atmosphere has a thermal conductivity exceeding 0.03 Wmxe2x88x921Kxe2x88x921.
Preferably, the packaging atmosphere includes helium. More preferably, the packaging atmosphere including helium in a proportion taken from the group of: at least 5% wt; 5% wt to 10% wt; 50% wt to 75% wt; more than 75% wt; 100% wt.
Preferably, the thermal efficiency of the packaging atmosphere exceeds the thermal efficiency of air by a factor of 1.5.
Preferably, the MEMS device is a MEMS mirror-type device.
The packaged MEMS may further include an atmospheric circulator. The atmospheric circulator may be a fan. The MEMS device may be packaged so that the packaging atmosphere is drawn over surfaces of the MEMS device to increase the heat flow from the surfaces to the packaging atmosphere.
The MEMS device-is sealed within an atmosphere having a desired level of thermal conductivity. By selecting a packaging atmosphere surrounding the MEMS device having a high conductivity, heat is transported more rapidly from the surfaces of the mirror and its support to the surrounding atmosphere than in known packaging methods.
Whereas conventionally MEMS-type devices are immersed in nitrogen-type atmospheres so that moisture content can be controlled, the invention immerses the MEMS-type device in alternative atmospheric environment. The composition of the atmosphere is selected to provide a desired thermal property, for example, to increase heat flow from the mirror surfaces and/or supports.
A fourth aspect of the invention relates to an optical switch having a packaged MEMS mirror device, the packaged MEMS mirror device comprising a MEMS mirror device surrounded by a packaging atmosphere and sealed within the packaging atmosphere by a packaging material, the packaging atmosphere having a thermal conductivity exceeding the thermal conductivity of air.
It is advantageous for any MEMS device having thermally dependent characteristics affecting its operational performance to be packaged within a suitably thermally conductive atmosphere. By selecting a suitable packaging atmosphere, the operational performance of the device can be improved and the temperature range for operating the device increased. The operational lifetime of the device can be extended, and deterioration caused by the thermal dependence of certain characteristics of the device alleviated.